1. Field of the Invention
The present invention relates to an elastic roller adapted for image forming apparatus such as electrophotographic copying apparatus and particularly suitale for use in liquid developing part or liquid cleaning part in an image forming process.
More particularly, the present invention is directed to improvements in such elastic roller and also to a method of making the improved elastic roller.
In image forming apparatus, for example, in an electrophotgraphic copying machine of the type in which liquid developer is used, there are conventionally provided elastic rollers for such stations of developing, liquid squeezing, cleaning etc. As one of elastic rollers developed for this purpose, Japanese Patent Application Laid Open No. 55644/1977 has already disclosed such type of elastic roller in which the spongy layer retentive of liquid of an elastic roller is covered by an additional flexible member of liquid permeability such as network member. When such elastic roller is rotated while contacting with a surface of rigid member such as photosensitive member or insulting member, a desired nip can be provided therebetween and liquid can be squeezed out from the liquid retentive layer of the roller or absorbed into the layer depending upon the elastic deformation of the roller. It is acknowledged in the art that such type of elastic rollers can be used advantageously in developing station, liquid squeezing part and cleaning station of electrophotographic copying apparatus.
2. Description of the Prior Art
While the above mentioned type of elastic roller according to the prior art as disclosed in Japanese Patent Application Laid Open No. 55644/1977 has various advantages, it involves a problem in assembling and manufacturing thereof.
As described above, the known elastic roller comprises an additional outer layer of network such as wire net provided on the circumference of the inner liquid retentive spongy layer of the roller. In assembling the outer network layer on the inner spongy layer there arises a difficulty to establish a close and uniform contact between the outer and inner layers without any irregularity of covering. This is attainable only by employing an extremely high standard of technique. In particular, when such elastic roller is to be incorporated into an electrophotographic copying machine, it is essential to lay the outer layer on the inner layer uniformly and tightly. Otherwise, a crease may be formed on the outer circumferential surface of the roller. In fact, hitherto, such troubles have been often observed in the copying machine that during operation and when the roller is brought into contact with the surface of a rigid member, that the elastic roller gets creased at its outer layer, that is, in the network. Also, it has been found that the development of creases in the elastic roller during operation is most remarkable at the end portions of the roller.
Treatment of end portions of the outer layer network at the both edges of the roller is also very important for performance and durability of the roller. If the end portions of the network are remained exposed at the both edges of the elastic roller, then the network becomes frayed gradually starting from the end portions during operation. After once frayed, the elastic roller no longer functions properly and there is caused irregularity along the length of the roller in liquid squeezing and absorbing action.
The above mentioned problems of creasing on the roller surface and irregularity of roller action along the length of the elastic roller may be partly attributable to the different in flexibility or hardness between the outer and inner layers. At all events, use of the elastic roller having such defects will bring forth various troubles in making copies. When such an elastic roller is used in the developing step, irregularity of developing performance may be caused by creases formed on the roller surface and by the irregularity of liquid squeezing and absorbing action along the roller length. If the roller is used to remove any excess of liquid developer, also irregularity of squeezed developer may be caused. When used as a cleaning roller, no uniform cleaning can be expected with such elastic roller and in the next step of electric charging also irregularity of charge may be caused thereby.
In addition to the above problems, the known elastic roller has another important problem. This problem is that during a long use of the roller the end portions of the roller become tapered towards the outside and liquid developer unnecessarily flows along the tapered end portions to the surface of a member with which the roller is in contact under pressure, for example, to the surface of photosensitive drum. This unfavourable flow of liquid developer onto the drum surface along the tapered end portions of the elastic roller makes dirty the visualized or developed images and in the worst case makes dirty also transfer sheets finally.
Until now, there has not yet been proposed any effective solution to the above mentioned problems involved in the known elastic roller.